The present invention relates to the recovery of lactose from liquids formed as by-products in the dairy industry and related industries.
More specificly, the invention relates to a process and a plant for converting lactose-containing liquids into easy to handle particulate products having non-caking and free-flowing characteristics.
In the present specification and the attached claims the term xe2x80x9clactose-containing liquidsxe2x80x9d is used as comprising solutions, wherein all lactose is in a dissolved state, as well as slurries of lactose crystals in lactose solutions.
Lactose is a disaccharide, which in solid state exists in at least three forms, viz. xcex1-lactose, crystallizing as the monohydrate from aqueous solutions at temperatures below 93.5xc2x0 C., xcex2-lactose, and amorphous lactose.
Of these three forms, the xcex1-lactose monohydrate is preferred since it when relatively pure forms stable, hard, and non-hygroscopic crystals.
In contrast thereto, especially the amorphous form of lactose is hygroscopic, and the presence of even minor amounts of the amorphous form on crystals of xcex1-lactose monohydrate may impair the non-caking and free-flowing characteristics of the latter.
Lactose is present in a concentration of approximately 5% in cow milk, which is the dominating source therefore.
Nearly all cheese-making processes have a by-product, whey, which is an aqueous solution, which, besides nearly all the lactose originally present in the milk, contains some fat, protein, and inorganic salts. The term xe2x80x9cwheyxe2x80x9d is here applied in a broad sense, comprising cheese whey, rennet casein whey, acid whey, and salty whey.
With the primary purpose of recovering protein values from the whey or from milk, this is often subjected to an ultra-filtration, whereby the proteins are obtained as retentate, whereas the lactose and most of the minerals remain dissolved in the water passing through the filter membrane. This aqueous solution of lactose with highly reduced protein consent is simply termed xe2x80x9cpermeatexe2x80x9d. This is the lactose source preferred in connection with the present invention although other whey-related products come into consideration.
Before used as starting material in the exploitation of the present invention, such permeate or related whey products may be subjected to a purification, e.g. a demineralization, and it is, preferably by means of vacuum evaporation, concentrated to a dry solids content of 40-75% by weight.
The main applications for lactose are as sweetener for ice cream mixes, for baking applications, and as component of animal feed but substantial amounts are also used as nutrient in culture media for micro-organisms in the bio-technical industries. For the baking industry it is, inter alia used with the purpose of obtaining a desired brown colour of the bread crust.
Although these various fields of application have different requirements as to purity and visual appearance of the lactose, it is for all applications desired to use the lactose as a particulate non-sticking, non-hygroscopic, and non-caking free-flowing product.
Several processes have been suggested and used for converting lactose in whey or whey-derive products into a particulate material.
However, a relatively fast drying process as obtained by spray drying results in particles, in which only a part of the lactose is present as xcex1-lactose monohydrate, whereas the remaining part thereof is present in a form which makes the particles sticky and hygroscopic.
Thus, a certain rest time is necessary before the cooled product resulting from the drying process can form free-flowing particles.
However, even after such treatment the resulting particles show tendency of hygroscopicity and caking.
This principle of fast drying combined with a subsequent rest is utilized in the FILTERMAT(copyright) process (Niro A/S) disclosed in U.S. Pat. No. 4,351,849 and also in the process disclosed in U.S. Pat. No. 5,006,204 (Assignee: A/S Niro Atomizer). In this last-mentioned process, a pre-crystallization is used before the spray drying, and the spray dried material rests on a rotating disc before after-drying in a fluidized bed.
A different concept is used in U.S. Pat. No. 6,335,045, according to which the lactose-containing concentrate is heated to a temperature above the crystallization temperature and thereafter subjected to a flash process resulting in a concentrate of higher dry solids content than the starting concentrate, which concentrate is subsequently cooled to induce crystallization. Afterwards, the crystallized concentrate can be dried in a spin-flash dryer. However, even with this rather complicated process, the results are not always satisfactory in terms of caking properties.
In the considerations preceding the present invention, we assumed that if a higher percentage of the lactose in the particulate product were present as the xcex1-lactose monohydrate form, the properties in the above-mentioned respects would be improved, and this turned out to be correct.
Highly concentrated aqueous lactose solutions have high viscosity, and we assumed that this could be an important reason why the crystallization of xcex1-lactose monohydrate in the prior art processes has been incomplete.
The invention is based on the recognition that it is possible by intensive agitation of the concentrate while it is being further concentrated by evaporation, due to the pseudo-plastic and thixotropic properties of the concentrate, to reduce the viscosity thereof to an extent, which highly promotes crystallization. The beneficial effect of this decrease of viscosity on crystallization more than compensates for any negative effect on the crystallization process caused by the intensive agitation. Thereby it is achieved that formation and growth of crystals occur simultaneously with the evaporation of the concentrate. Thereby, the viscosity is lowered, not only due to the pseudo-plastic and thixotropic characteristics of the concentrate mentioned but also because an excessive viscosity increasing super-saturation would be prevented.
It should be observed that generally the effect of mechanical agitation on the crystallization process is somewhat unpredictable. Reference is made to Mullin and Raven: xe2x80x9cInfluence of mechanical agitation on the nucleation of some aqueous salt solutionsxe2x80x9d, Nature, Vol. 195, page 35-38 (1962). According to said paper, nucleation, which is a prerequisite for crystallization, is generally higher in a moderately agitated liquid than under quiescent conditions but with increasing intensity of agitation, nucleation decreased and at still further intensities increased again and finally some decrease was observed. In said paper it is proposed that this unpredictable effect is a combined result of the influence of the agitation on the diffusion and the attrition.
In most commercial crystallization processes where relatively large crystals are desired, such as in the sugar industry, crystallization is performed at rather gentle agitation of the crystallizing medium, e.g. by using so-called crystallization cradles. An intensive agitation is regarded as harmful, not only due to attrition but also because it may result in the formation of too many crystallization embryos causing too small and uneven crystals.
However, as mentioned above, it has turned out that due to the pseudo-plastic and thixotropic properties of the lactose concentrates under evaporation an improved crystallization result is obtained by using a mechanical agitation of high shear rate.
The present invention thus relates to a method for evaporative concentration and crystallization of a viscous lactose-containing aqueous liquid, comprising:
(a) introducing the lactose-containing aqueous liquid having a dry solids content of 40-75% by weight into a zone and therein subjecting it to the following simultaneous measures:
i) progressive heating at a temperature above 40xc2x0 C. but below the maximum temperature for crystallization of xcex1-lactose monohydrate from the liquid;
(ii) removal of vapour evaporated from the liquid; and
(iii) mechanical agitation able to provide a crystallization promoting decrease of the viscosity of the liquid with crystals formed and suspended therein;
to progressively concentrate and simultaneously crystallize the agitated liquid, and
(b) recovering the resulting slurry from said zone at a total solids content above 75% for cooling and optional disintegration and further drying.
By this process, optimal conditions for crystallization of xcex1-lactose monohydrate are achieved.
The mechanical agitation drastically decreases the viscosity of the liquid being evaporated, whereby lactose molecules in the only slightly super-saturated liquid easily diffuse to the surface of crystals and nuclei therefore to promote crystal growth meaning that the content of dissolved lactose and thus the degree of super-saturation of the liquid decreases.
Also the simultaneous progressive evaporation and crystallization ensures the existence of optimal conditions for crystallization over a prolonged period and over a broad range of temperature levels of the liquid.
The above measure xe2x80x9cremoval of vapour evaporated from the liquidxe2x80x9d should be construed in a broad sense as covering not only active steps, such as application of reduced or increased pressure, but also the mere ensuring that such vapour can be vented.
The process is preferably performed as a continuous operation, wherein
said lactose-containing aqueous liquid is introduced into one end of an elongated essentially horizontal zone;
the heating in measure (i) is carried out by supplying a heating medium to a jacket surrounding at least a portion of said zone;
said vapour evaporated from the liquid is in measure (ii) removed by purging said zone by passing an air flow therethrough;
the agitation in measure (iii) is performed by rotating an essentially horizontal shaft carrying agitation means to agitate the total volume of liquid present in said zone; and
said slurry is recovered from the end of the elongated zone opposite to said one end.
In a typical embodiment of the process, the lactose-containing aqueous liquid used as starting material is selected from the group consisting of concentrates of permeate formed by ultrafiltration of sweet or acid whey or milk, concentrates of whey or of demineralized whey, mother-liquor from lactose recovery, and lactose slush.
In the above-described continuous operation, the heating is preferably performed by supplying steam to the jacket mentioned. If the liquid to be treated is permeate concentrate coming directly from a conventional evaporator its content of dry solids can typically be 55-60% and its temperature typically 65xc2x0 C.
By passage through said zone, the amount of total dry solids increases to e.g. 87% and the amount of crystallized xcex1-lactose monohydrate to 85% of the lactose.
Due to the previous intensive agitation of this pseudo-plastic and thixotropic mass, by the recovering from said zone it forms a gruel, which is still transportable, e.g. pumpable.
The above-mentioned cooling and optional disintegration and further drying can for instance be accomplished by passing said gruel through an auger apparatus having cooling surfaces, and subsequently through a disintegrating and drying device.
However, the cooling may be a natural cooling or a forced cooling.
The invention further comprises a plant suitable for carrying out the above-described process.
Thus, in this aspect the invention deals with a plant for converting an aqueous lactose-containing liquid into free-flowing particles comprising:
(a) a first device having (i) an elongated cylindrical horizontal housing of essentially circular vertical cross-section, (ii) an inlet for lactose-containing liquid at one end of said housing, (iii) a rotatable horizontal shaft axially in said housing, (iv) paddles radially extending from the shaft towards the cylindrical wall of the housing, (v) a heating jacket surrounding at least partially said housing, (vi) openings at each end of the housing for removing vapours and, (vii) an outlet in the housing for lactose-containing gruel near the end opposite to said one end,
(b) a cooling device connected to said outlet end designed as an auger having at least one screw end being provided with a jacket and/or hollow screw shaft(s) for receiving a cooling medium, and
(c) a disintegrating and drying device connected to said cooling device.
The device defined under (a) can be rather similar to an apparatus disclosed in U.S. Pat. No. 3,425,135. In said disclosure, the apparatus is described as an apparatus for processing solids and all the examples illustrate drying of such.
The residence time in this apparatus can be adjusted by setting the padres in a backward pitch. Thereby sufficient time for an almost complete crystallization can be obtained when performing the process of the present invention. An apparatus of similar construction is disclosed in U.S. Pat. No. 5,271,163, wherein it is described as suitable for flowable materials. However, this last-mentioned apparatus is specific by having means for introduced gas through nozzles in the rotating parts with a view of spreading the material to be treated over the walls of the housing. No applications akin to lactose crystallization seem to be mentioned in said US patent.
The process and the plant according to the invention are further elucidated below by reference to the drawing.